1. Field of the Invention
The present invention relates to an improvement in a method for producing thallium type superconductors. More particularly, it relates to a method for producing superconducting compound oxides containing thallium (Tl), such as a Tl--Ba--Ca--Cu type oxide, which have a high critical temperature.
2. Description of the Related Art
Superconductivity is a phenomenon which is described as a kind of phase change of electrons under which an electric resistance becomes zero and perfect diamagnetism is observed. When superconductor technology is applied to electric power transmission, power loss of about 7%, which is experienced in electric power transmission systems today, can be greatly reduced. Development of superconductor technology is also expected in the field of measurement and in the field of medical treatment such as NMR, .pi. neutron medical treatment or high-energy physical experiments. In electromagnet devices for generating strong magnetic fields, superconductor technology is expected to accelerate development of the technology of fusion power generation, MHD power generation, magnetic levitation trains and magnetically propelled ships.
The critical temperature "Tc" of superconductors, however, had not exceeded 23.2K exhibited in Nb.sub.3 Ge which was the highest Tc reported over the past ten years.
The existence of new types of superconducting materials having much higher Tc was revealed by Bednorz and Muller, who discovered a new oxide type superconductor in 1986 (Z. Phys. B64, 1986 p. 189).
The new compound oxide type superconductor discovered by Bednorz and Muller is represented by [La, Sr].sub.2 CuO.sub.4, which is referred to as a K.sub.2 NiF.sub.4 -type oxide, having a crystal structure which is similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type compound oxides have exhibited a higher Tc on the order of 30K which are much higher than previously known superconducting materials.
It was also reported in February 1987 that C. W. Chu et al. discovered, in the United States of America, another superconducting material of the so called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having a critical temperature of about 90K (Physical Review letters, Vol. 58, No. 9, p. 908).
Other new superconducting materials have been reported recently, such as a compound oxide of a Bi--Sr--Ca--Cu--O system reported by Maeda et al (Japanese Journal of Applied Physics. Vol. 27, No. 2, p. 1209 to 1210) and a Tl--Ba--Ca--Cu--O system, which exhibit a Tc in excess of 100K (Hermann et al. Appl. Phys. Lett. 52 (20) p. 1738) and which are chemically more stable than the abovementioned YBCO type compound oxide or the like. Hence, the possibility of actual utilization of the high Tc superconductors has burst onto the scene.
A variety of compound oxides which show high critical temperatures have subsequently been reported. Among them, thallium (Tl) type compound oxides have the significant advantage that high Tc superconductors having a T.sub.c higher than 100K can be realized without using rare earth elements as a material, so that the production cost can be reduced.
The above-mentioned oxide type superconducting materials can be prepared in a bulk form as a sintered block obtained by sintering a powder mixture of oxides or carbonates of constituent metal elements, or can be deposited on a substrate in the form of a thin film by a physical vapor deposition (PVD) technique or a chemical vapor deposition (CVD) technique.
In the case of production of thallium type oxide superconductors, however, there is a special problem because thallium (Tl) is a very volatile element and toxic to humans. Therefore, it is necessary to adopt a special sintering technique when the thallium type compound oxides are produced by sintering a material powder mixture.
Heretofore, when the material powder mixture for thallium type compound oxides have been sintered, it has been usual practice to wrap the material powder mixture in a foil made of gold in order to prevent volatile thallium vapour from escaping. However, it has proven to be difficult to completely suppress the escape of thallium vapour out of the gold foil, so that the atomic ratios of the component elements in the resulting sintered mass will deviate from desired values. In fact, it is difficult to produce a sintered mass having consistent quality and a stable high-value T.sub.c by this process.
It has also been proposed to carry out the sintering of material powder mixture for the thallium type compound oxides in a pipe made of gold ("Science" vol. 240, page 631 to 634, Apr. 4, 1988). As a variation, it has been proposed to sinter the material powder mixture wrapped in a gold foil in a sealed pipe made of quartz [Report submitted to "Phys. Review Letter" by S. S. P. Parkin et al. RJ 6147 (60857) 3/18/88].
These methods, however, have the problem that oxygen supply to the material powder mixture during the sintering operation is impossible or difficult, so that the oxygen content in the resulting sintered mass will be insufficient. In fact, the products obtained by these methods show rather poor superconducting property. Further, there is a danger of breakage of the pipe if the pipe is sealed in an air-tight manner.
An object of the present invention is to overcome the problem exhibited in the prior art process and to provide an improved method for producing thallium (Tl) type compound oxide superconductors.